feat(monitor): active push loop replacing standalone monitor

Adds a periodic POST loop to <backend>/monitor/server/heartbeat so
HF plugin can take over the standalone harborforge-monitor daemon's
job — same X-API-Key header, same flat telemetry shape (cpu_pct /
mem_pct / disk_pct / swap_pct / load_avg / uptime_seconds /
plugin_version / agents[]). HF backend stays unchanged.

Config: monitor_push_enabled (default false; opt-in to avoid surprise
heartbeats from existing deployments), monitor_push_interval_seconds
(default 30), reuses apiKey for the X-API-Key header. Lift the
container's HF_MONITER_API_KEY into config.apiKey, flip
monitor_push_enabled true, then docker rm -f the container — DB
last_seen_at keeps advancing under the plugin's loop.

Collector grew swap + cpu sampling (two reads of /proc/stat over a
1-second window when SampleCPU=true). Bridge endpoint stays cheap
(SampleCPU=false on demand); push loop is the only caller paying the
sampling cost.

E2E in sim: monitor_push_enabled=true + apiKey from injected
MonitoredServer row → server_states.last_seen_at advances exactly
every interval_seconds (10s configured, 10s observed). cpu/mem/disk/
swap_pct all populate correctly.

scripts/install.sh

@@ -37,12 +37,16 @@ Next steps:
        "harbor-forge"
   2. Write ${PLUGIN_DIR}/config.json — sample:
      {
-       "backendUrl": "https://monitor.hangman-lab.top",
+       "backendUrl": "https://hf-api.hangman-lab.top",
        "identifier": "server-t3",
-       "apiKey": "g1_xxx",
-       "monitor_port": 9100,
+       "apiKey": "<copy from HF_MONITER_API_KEY>",
+       "monitor_push_enabled": true,
+       "monitor_push_interval_seconds": 30,
+       "monitor_port": 0,
        "calendar_enabled": true,
        "calendar_heartbeat_interval_seconds": 30
      }
   3. Restart the host:  systemctl --user restart plexum
+  4. Verify push is landing (DB last_seen_at advancing) and then
+     remove the standalone harborforge-monitor container.
 EOF